The oil supply systems known from the prior art are used to lubricate the radial and axial bearings of gas turbines, for example. The rotor of the gas turbine is supported in the bearings of the gas turbine and generally rotates at a speed of 3000 rpm or 3600 rpm during operation of the gas turbine. A generator connected to the rotor then converts the energy supplied to the gas turbine in the form of fossil fuel into electric energy. In this arrangement, the bearings generally have a lift oil system and a lubricating oil system. The lift oil is required in the radial bearing in order to enable the gas turbine to be started or shut down at a relatively low rotor speed through hydrostatic lubrication without the occurrence of damage in the bearings and to the rotor. The lubricating oil system is used to supply the bearings in rated operation, i.e. at relatively high speeds, which allow hydrodynamic lubrication. The lift oil system and the lubricating oil system often have a common oil tank, in which the oil returned from the bearings is stored. The oil stored in the oil tank after use is generally at a temperature of 80° C. to 90° C. owing to the friction to which it is subjected in the bearing and owing to the environment in which it is used. The oil tank is generally dimensioned in such a way that the quantity of lubricating oil fed back can remain in it long enough for the air dissolved therein to escape to a sufficient extent before the oil is once again fed to the lift oil and lubricating oil systems.
A heat exchanger is furthermore provided in the inlet conduit of the lubricating oil system in order to cool the lubricating oil taken from the oil tank at a temperature of about 70° C. to an inlet temperature of about 40° C.-50° C. The heat exchanger is designed as an air-cooled or water-cooled heat exchanger, for example, and may be fitted with a cooling fan. A system of this kind is known from DE 43 04 482 A1, for example.
However, there is the disadvantage that cooling of the lubricating oil to 50° C., the maximum permissible inlet temperature, can no longer be guaranteed with the air-cooled heat exchanger in the case of very warm ambient temperatures of about 50° C. and, in some rare cases, even higher, and that a water-cooled heat exchanger is generally not available in pure gas turbine power plants. In such cases, the inlet temperature of the lubricating oil can only be reduced to a value of 50° C. (or less) if a compression-type refrigerating machine is inserted in the feed conduit in addition to the air-cooled heat exchanger. However, this conversion is associated with higher costs and impairs the overall efficiency of the plant due to the increase in its own consumption through the use of the compressor of the refrigerating machine. Moreover, the use of the additional component—the compression-type refrigerating machine—compromises the availability of the gas turbine. Another limiting boundary condition in addition is that the lift oil pump provided in the lift oil circuit must not be operated at an oil temperature of 73° C. or above. This is due to the fact that exceeding the oil temperature leads to an excessively low oil viscosity (e.g. less than 10 cSt), which no longer guarantees safe and reliable operation of the lift oil pump. Although there would also be the possibility of using pumps suitable for these conditions, said pumps would be significantly more expensive to procure.
Moreover, there is the risk, when the oil temperature of about 75° C. is exceeded, that the bearings will no longer be adequately cooled in operation. This too would compromise safe operation of the gas turbine.
U.S. Pat. No. 4,105,093 has furthermore disclosed the practice of using pressurized reservoirs in lubricating oil systems. This reservoir is supplied with cooled hydraulic fluid by means of a pump and a heat exchanger. The disadvantage with this embodiment is that the pumps are located in the relatively warm tank region, and therefore the abovementioned problems as regards viscosity can also occur there. Moreover, U.S. Pat. No. 5,611,411 discloses an uncooled lubricating oil system for a turbine generator, the vertically divided tank of which allows lubricant equalization by means of a flap.